1. Field of the Invention
The present invention relates to devices for inspecting the in-bundle region of a steam generator above the tubesheet. The in-bundle region is comprised of two hemispherical regions extending from the second row of tubes beyond the last row of tubes and into the annulus on each side of the tubelane. Providing an ability to remotely inspect the in-bundle region on the top of the tubesheet is an important element of any steam generator maintenance program. The principle reasons for inspecting the in-bundle region include the need for monitoring the effectiveness of sludge lancing operations, for determining the trend fouling of the tubesheet and tube surfaces, for confirming potential loose parts identified through the eddy current inspection program and for searching and retrieving foreign objects.
2. Description of Related Art
Devices for inspecting the exterior walls of conduits such as the heat exchanger tubes of a steam generator are known in the prior art, for example in U.S. Pat. Nos. 5,982,839 and 5,963,030, the disclosures of which are hereby incorporated by reference. Additionally, devices for inspecting the interior walls of heat exchanger tubes of a steam generator are known in the prior art, for example in U.S. Pat. Nos. 6,357,310, 5,254,944, 5,313,838, 4,901,578 and 3,994,173, the disclosures of which are hereby incorporated by reference. Generally, such devices have included ultrasonic probes, and/or eddy current probes to inspect the walls of tubes for flaws, as shown in U.S. Pat. Nos. 5,105,876, 5,025,215 and 4,955,235, the disclosures of which are also hereby incorporated by reference. However, before the purpose and operation of such inspecting devices may be fully appreciated, some knowledge of the structure, operation and corrosion degradation problems associated with the heat exchanger tubes in steam generators is necessary.
Nuclear steam generators are comprised of three principal parts, including a secondary side, a tubesheet, and a primary side which circulates water heated from a nuclear reactor. The secondary side of the generator includes a plurality of U-shaped heat exchanger tubes, as well as an inlet for admitting a flow of water. The inlet and outlet ends of the U-shaped tubes within the secondary side of the generator are mounted in the tubesheet which hydraulically isolates the primary side of the generator from the secondary side. The primary side in turn includes a divider sheet which hydraulically isolates the inlet ends of the U-shaped tubes from the outlet ends. Hot water flowing from the nuclear reactor is admitted into the section of the primary side containing all of the inlet ends of the U-shaped tubes. This hot water flows through these inlets, up through the tubesheet, and circulates around the U-shaped tubes which extend within the secondary side of the generator. This water from the reactor transfers its heat through the walls of the U-shaped heat exchanger tubes to the non-radioactive feedwater flowing through the secondary side of the generator, thereby converting feedwater to non-radioactive steam which in turn powers the turbines of an electric generator. After the water from the reactor circulates through the U-shaped tubes, it flows back through the tubesheet, through the outlets of the U-shaped tubes, and into the outlet section of the primary side, where it is recirculated back to the nuclear reactor.
Over a period of time, sludge may accumulate in the annular spaces between the heat exchanger tubes and the tubesheet or support plates which surround them. Despite the fact that the heat exchanger tubes are formed from a corrosion-resistant alloy such as Inconel RTM™, these corrosive chemicals, in combination with the hot water which flows around such tubes, may cause a number of different forms of corrosion degradation. If unchecked, such corrosion may ultimately result in fissures in the walls of the tubes, which can cause water leakage through the walls of these tubes. In addition to reducing the efficiency of the steam generator as a whole, such leakage may cause radioactive elements carried by the water from the primary side of the generator to contaminate the non-radioactive water in the secondary side, thereby rendering the steam created by the generator undesirably radioactive.
In order to prevent such corrosion degradation from creating leaks in the heat exchanger tubes, a number of maintenance procedures have been developed, such as “sleeving” and “plugging” of badly corroded tubes. In order to repair tubes at the earliest possible states of corrosion and to thereby avoid the necessity of plugging tubes, both elongate ultrasonic probes and eddy current probes have been used to inspect the exteriror and interior walls of such heat exchanger tubes for degradation which indicates the beginning of a corrosive pattern.
Unfortunately, each type of external and internal inspection device is limited in its ability to be easily and efficiently positioned at the site of the small-diameter tubes in a nuclear steam generator while still being capable of perfectly informing the operator of the size, shape and type of a corrosion-induced flaw in a small-diameter tube of a nuclear steam generator. Probes attached to elongate feed cables of various designs are well known for inspecting the interior of a tube, as illustrated in U.S. Pat. Nos. 5,279,168, 5,174,165 and 5,174,164, the disclosures of which are hereby incorporated by reference. However, each of these systems involve feed and cable assemblies which are elaborate and cumbersome to install at the site and do not enable inspection of the exterior of the tubing of an in-bundle region. Further, the underlying need driving remote inspection operation of small diameter tubes in a nuclear steam generator is the reduction of human radiation exposure. Manual operation of inspection and retrieval devices from in front of steam generator hand-holes is radiation dose intensive work. As a consequence, anticipated high radiation exposure often causes service utilities to decide to leave foreign objects in their steam generators and to entirely exclude the top of tubesheet visual inspections from their inspection plans. Further, with the implementation of risk informed eddy current programs which allow extension of inspection intervals beyond one operating cycle, there is the necessity to satisfy regulatory concerns over loose part-induced tube leaks when skipping eddy current inspections for one or more cycles.
Clearly, there is a need for a remote, small-diameter tube inspecting device which is small enough to be used for inspecting the exterior of tubes in a tubesheet in the heat exchanger tubes of a nuclear steam generator which is capable of detecting flaws in the walls of these tubes with a higher degree of accuracy and reliability than previously achieved. Ideally, such an inspection device would be capable of being easily assembled at the work site, be capable of being quickly and efficiently positioned between the small-diameter tubes, and be capable of resolving all types of flaws, regardless of shape or orientation, as well as areas where the walls have been uniformly thinned by corrosion. Finally, such a device should be reliable in operation, and relatively easy to manufacture from commercially available components. These wide variety of constraints for remote inspection has generated many conflicting functional requirements which has led to fairly complex remote inspection devices as illustrated above.